In the manufacture of integrated circuits by lithographic methods exposure masks must be aligned (or registered) relative to structures on the wafer with an accuracy that is only a fraction of the smallest structural detail in the mask. In present photolithographic methods with smallest line widths on the order of 1 micron alignment accuracies of 0.3 micron can be achieved with manual or automatic alignment methods using suitably shaped alignment marks on mask and wafer.
The further decrease of line width to some tenths of a micrometer which is expected from corpuscular beam and X-ray lithography over the next years will thus require alignment accuracies in the order of nanometers. Additionally, alignment must be performed in short times (typically less than a second) for high throughput in the production line, in particular when large wafers are exposed in the step-and-repeat-mode where each position requires its own alignment. Conventional visual methods cannot meet these requirements for accuracy and speed.
One approach that has been discussed in the art to achieve high accuracy uses optical gratings as registration marks on mask and wafer and derives the registration control signal from light diffracted at these gratings. Accuracies in the nanometer range can then be achieved if the evaluation process is based on phase sensitive methods as described in U.S. Pat. No. 4,577,968 (corresponding to European Pat. No. 45321) or in an article by G. Makosch and B. Solf entitled "Surface profiling by electro-optical phase measurements" (Proc. Soc. Photo-opt. Instrum. Eng., Vol. 316, pages 42-53, 1981). This prior system is, however, conceived for photolithographic printers of the projection type where an optical imaging system is interposed between mask and wafer. It thus cannot be used in proximity printing systems with mask-wafer separations on the order of between 50 and 100 microns.
Alignment techniques using optical gratings on mask and wafer in an x-ray exposure system of the proximity printing type have been described by A. Une, et al in an article entitled "A Highly Accurate Alignment Technique Using a Dual Grating" (Bull. Japan Soc. of Prec. Engg., Vol. 19, No. 1, pages 71 and 72, March 1985), and by J. Ito and T. Kanayama in an article entitled "A New Interferometric Displacement-Detection Method for Mask-to-Wafer Alignment Using Symmetrically-Arranged Three Grating" (Japanese Journal of Applied Physics, Vol. 25, No. 6, June 1986, pages L487-L489). These techniques evaluate the intensities of different diffraction orders and might therefore encounter difficulties in practical use when the wafer is covered with various layers in different processing steps that may change the reflectivity and give rise to interference effects disturbing the measured intensity.